Field of the Invention
The invention relates to methods of obtaining diagnostic information relating to an industrial process. An example of an industrial process for which the method has been developed is a lithographic process, which includes one or more steps of transferring a pattern from a patterning device onto a substrate using a lithographic apparatus. The invention further relates, to a diagnostic apparatus, device manufacturing method, to a controller for an industrial process and to a computer program products for causing a data processing apparatus to implement the methods and apparatus described
Related Art
A lithographic process is one in which a lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate, after which various processing chemical and/or physical processing steps work through the pattern to create functional features of a complex product. The accurate placement of patterns on the substrate is a chief challenge for reducing the size of circuit components and other products that may be produced by lithography. In particular, the challenge of measuring accurately the features on a substrate which have already been laid down is a critical step in being able to position successive layers of features in superposition accurately enough to produce working devices with a high yield. So-called overlay should, in general, be achieved within a few tens of nanometers in today's sub-micron semiconductor devices, down to a few nanometers in the most critical layers.
Consequently, modern lithography apparatuses involve extensive measurement or ‘mapping’ operations prior to the step of actually exposing or otherwise patterning the substrate at a target location. So-called advanced alignment models have been and continue to be developed to model and correct more accurately non-linear distortions of the wafer ‘grid’ that are caused by processing steps and/or by the lithographic apparatus itself. Not all distortions are correctable, however, and it remains important to trace and eliminate as many causes of such distortions as possible.
Modern lithographic process and products are so complex that issues due to processing are difficult to trace back to the root cause. Overlay and alignment residuals typically show patterns over the wafer (of the process and/or litho tool). This may be interpreted as a non-correctable quantity with respect to a predefined model, while visual inspection and detailed analysis of the fingerprint may give an indication of causes and correction strategies. The spatial pattern in the fingerprint is not used to quantify the fingerprint, nor the observation that multiple causes may show up simultaneously in the apparent fingerprint. Overlay measurements are not generally available for each individual wafer, and the relation to the processing history and context is not generally known or used. Furthermore, it is difficult and time-consuming to make a list of all possible sources of spatial variation for the machine and process at hand.
Aside from the problem of identifying causes of processing errors, process performance monitoring systems have been implement which allow measurement of performance parameters to be made from processed products, which then are used to calculate corrections for use in processing subsequent products. A limitation with current performance monitoring systems is that there is a compromise between the amount of time and equipment dedicated to performance monitoring, and the speed and accuracy with which corrections can be implemented.
It would also be desirable to have a performance monitoring system that can detect problems at an early stage, rather than only detecting them when some performance parameter exceeds permissible limits.